The invention relates to a method for producing an object on which an exterior layer is applied by thermal spraying, followed by a heat treatment, and to an object, in particular a drill bit, obtained pursuant to this method.
Such a method is disclosed in British Pat. No. 1,367,762. In application of the method described above to objects wherein it is required that the exterior layer applied be capable, in operation, of withstanding great variable forces, for example, that it must be resistant to wear, however, it happens that this layer sometimes chips off, thus shortening the life of the object obtained.
The invention accordingly procures a method of the type mentioned at the beginning, characterized in that on a core member is applied, by cold isostatic compacting, a layer of a suitable powder material, followed by sintering, after which the exterior layer, which is a wear-resistant layer, is applied and then the structure thus obtained is isostatically compacted hot.
It has been found that a suitable powder material for this purpose is a nickel-containing alloy steel powder with preferably 3.5% nickel therein.
The invention in addition procures a drill bit with cutting teeth provided with a wear-resistant layer, for drilling in rock.
For the performance of a method pursuant to the invention a supply of powder material is introduced into a rubber mold and distributed, after which the core member, which is usually a type of steel suitable for a bearing, is placed in the powder, following which the powder is pressed on. The core member may alternatively be placed in the mold first, after which the powder material is introduced and pressed on. The mold is closed and is then isostatically compacted cold until a coherent member having a density of approximately 90% is obtained. The compact removed from the mold is then sintered in a furnace. After cooling the sintered object is coated with a wear-resistant layer by thermal spraying, for example plasma spraying, after which the structure thus obtained is isostatically compacted hot. This hot isostatic compacting may be done by inserting the entire object in a thin-walled deep-drawn vessel or container of low-carbon steel having a wall thickness of approximately 0.5 mm, filled with a ceramic powder. This vessel is then heated and placed under pressure on all sides. After hot isostatic compacting the object may be readily separated from the surrounding ceramic mass and cleaned by sand blasting. This method proves to procure components with accurately shaped dimensions comparable to those of a forged product.
When a drill bit for rock is produced in this fashion, after sintering not the entire surface of the cutting teeth but only the parts thereof which come directly into contact with the rock are coated with the wear-resistant layer by thermal spraying. Following the selective application of the wear-resistant layer the preformed drill bit is subjected in its entirety to hot isostatic compacting, as described above.